During production of oil, it may become necessary to perform maintenance work in a well or to open a production well. Such well work is known as well intervention. A production casing is arranged inside the well, which is closed by a well head in its upper end. The well head may be situated on land, on an oil rig or on the seabed below water.
When a well head is situated on the seabed on deep water, well intervention is more complicated since connection to the well head is obtained under water.
In order to perform such subsea intervention operations, it is a known practice to lower an intervention module from a surface vessel onto the well head structure by means of a plurality of remotely operated vehicles (ROVs).
An intervention tool is placed in a lubricator before being submerged into the well. In order to lower and raise the tool into the well and supply the tool with electricity, the intervention tool is connected to a wireline at its top, which is fed through the lubricator from a winch. A lubricator is a long, high-pressure pipe fitted to the top of a well head, enabling tools to be put into a high-pressure well. The top of the lubricator includes a high-pressure grease injection section and sealing elements for sealing around the wireline. When a tool is placed in the lubricator, the lubricator is pressurised to wellbore pressure before the valves of the well head are opened and the tool is submerged into the well.
In order to seal around the wireline passing through the grease injection section of the lubricator, high-pressure grease is pumped into the surrounding annulus to effect a pressure-tight dynamic seal which is maintained during the operation by injecting more grease as required. A slight leakage of grease is normal, and the addition of fresh grease enables the consistency of the seal to be maintained at an effective level. In this way, grease leaks from the grease injection section into the sea during an intervention operation, which is not environmentally desirable. Due to the increasing awareness of the environment, there is a need for a more environmentally friendly solution.